1. Field of the Invention
The present invention relates to a method of non-destructively measuring the hydrogen concentration of a radioactive metallic material by using an electromagnetic acoustic resonance.
2. Description of the Related Art
Metallic materials are likely to be brittle due to high hydrogen concentration thereof. Thus, metallic materials in use in a nuclear power plant are tested for hydrogen absorption.
There are various hydrogen-measuring methods of measuring a very small quantity of hydrogen contained in a metallic material. One of those known hydrogen-measuring methods is a fusing method, which has the following steps. Less than 1 grams of specimen is sampled from an object of measurement. The specimen is fragmented, rinsed and dried, and the weight of the specimen is measured. Then, the specimen and a pure metal serving as a fusing agent are put in a refractory crucible, the specimen is melted at a temperature lower than the melting point of the specimen in an inert gas atmosphere to extract hydrogen together with other gases from the specimen. The extracted gas is collected in a container of a fixed volume including a thermal conductivity cell and the thermal conductivity of the gas is measured. The quantity of hydrogen contained in the gas is determined on the basis of the measured thermal conductivity of the gas, and the hydrogen concentration of the specimen is calculated on the basis of the quantity of hydrogen. Although this hydrogen measuring method is capable of determining hydrogen concentration in a high accuracy on the order of several tens parts per million, the hydrogen measuring method requires skilled work to sample the specimen from the measuring object and to fragment the specimen.
Another hydrogen measuring method cuts out a small, beam-shaped test piece from a measuring object and estimates the hydrogen concentration of the test piece by using the vibration attenuation characteristic of the test piece called internal friction featured by a hydride contained in the test piece.
The application of the foregoing destructive test method to testing a radioactive object made of metallic material is subject to many restrictions. The destructive test method must be carried out in an installation provided with various facilities meeting legal requirements on shields for analyzers, exhaust systems and such to ensure the perfect protection of operators participating in the test from exposure to radioactive rays and the perfect prevention of environmental radioactive contamination. The construction, maintenance and management of such an installation for analysis need very high costs.
An object to be inspected removed from a nuclear power plant must be transported to an installation for analysis by the following procedure, which requires much time and labor and high costs. An assembly including the object to be inspected is immersed in water in a pool annexed to a nuclear reactor to protect operators from exposure to radiation, and the object is removed from the assembly by means of a remote-controlled system. The object is put into an approved container called a cask proved to be capable of shielding radiation by a remote-controlled system. The cask containing the object is transported to an inspection installation under conditions meeting rules and provisions specified by relevant laws after obtaining permissions from government offices concerned and all the local governments on the route of transportation of the cask.
As mentioned above, the known test methods require much time and labor and high costs because the known test methods are of a destructive test system. The inventors of the present invention examined various non-destructive test methods that can be carried out in a pool annexed to a nuclear reactor.
An eddy-current test method is one of the nondestructive test methods examined. The eddy-current test method is applied widely to various tests including tests for determining a heat-treatment condition, measuring internal stress and evaluating structures. The eddy-current test method is mentioned in, for example, xe2x80x9cEddy-current Test IIIxe2x80x9d, MORI et al, The Japanese society for Non-Destructive Inspection (JSNDI), (1990). The eddy-current test method is applied not only to the detection of defects in objects, but also to various industrial purposes including the determination the quality of objects, the measurement of the thickness of films, measurement of dimensions of objects and the measurement of displacement. The electromagnetic characteristic, such as electric conductivity or magnetic permeability, of a metallic material changes slightly, depending on the hydride content of the metallic material. Therefore, the application of the eddy-current test method to the measurement of hydrogen concentration is theoretically feasible. However, the eddy-current test method is not sufficiently sensitive to the variation of hydrogen concentration, and is subject to many factors of noise generation, such as distance between the test coil and the specimen, irregularities in the surface of the specimen, electromagnetic properties of the specimen, measurement environment and measuring speed, and hence is not suitable for hydrogen concentration measurement.
Another known method examined is an ultrasonic test method, which measures the velocity and attenuation characteristic of a sound wave propagating through a metallic material with an ultrasonic probe, and determines the quantity of hydrogen contained in the metallic material on the basis of measured data. It is generally known as mentioned in, for example, xe2x80x9cCho-Ompa Gijutu Binranxe2x80x9d, SANEYOSHI et al, The Nikkan Kogyo Shimbun Ltd., (1978), that the velocity and attenuation characteristic of an ultrasonic wave propagating through the metallic material is dependent on features of the metallic material, such as crystal grain size, hardness and crystalline dislocation. However, since the ultrasonic test method uses the contact-type ultrasonic probe, a large amount of vibrational energy is absorbed by a coupling medium lying between the ultrasonic probe and the metallic material, and hence the accurate detection of a change in the vibrational energy absorbing characteristic of the metallic material due to the formation of a hydride in the metallic material cannot practically be achieved. Thus, the application of the ultrasonic test method to the measurement of the hydrogen concentration of a metallic object is infeasible.
An electromagnetic acoustic resonance method that uses an electromagnetic acoustic transducer (hereinafter, abbreviated to xe2x80x9cEMATxe2x80x9d) for the detection of the interior condition of an object has been recently developed. Techniques relating to this method are known and mentioned in, for example, xe2x80x9cHihakai Kensa (Journal of JSNDI)xe2x80x9d, The Japanese Society for Non-Destructive Inspection (JSNDI), Vol. 43, No. 12 (December, 1994). However, the application of the techniques to the nondestructive test for the determination of the hydrogen concentration of a metallic object has not been known.
The object of the present invention is to provide a method of non-destructively measuring a concentration of hydrogen contained in a radioactive test object of metallic material by using the electromagnetic acoustic resonance method.
According to the present invention, there is provided a method of measuring a concentration of hydrogen contained in a radioactive test object of metallic material. The method including the steps of: positioning an EMAT adjacent to the object, the EMAT including a magnet that creates a static magnetic field and a coil that creates a magnetic field variable with time; supplying a radio-frequency current to the coil of the EMAT, thereby generating an ultrasonic wave that is reflected for multiple reflection by opposite surfaces of the object; changing a frequency of the radio-frequency current, thereby finding a resonance frequency that make the object exhibit electromagnetic acoustic resonance by means of a detector connected to the coil of the EMAT; and determining the concentration of hydrogen contained in the object on the basis of an experimentally determined relation between the numerical value and the hydrogen concentration of the object, the numerical value being calculated by using an expression including the resonance frequency as a valuable.
According to the present invention, the hydrogen concentration of the test object can be readily determined nondestructively, upon highly accurately grasping the elastic-vibration characteristic, which is most sensitive to the variation of the hydrogen concentration of the metallic material.